Is the world portrayed in To Kill a Mockingbird by Harper Lee a fair one?
No, not at all. The world portrayed in To Kill a Mockingbird is deeply unfair. In common with much of the South at that time, racial prejudice is rife in Maycomb. African Americans aren't treated equally and are subjected to official discrimination. The criminal justice system is stacked against them, as we see in the case of Tom Robinson. Tom's convicted on a trumped-up charge of rape and assault, despite there being absolutely no evidence whatsoever of his guilt. He's been convicted purely and solely because he's an African American and his accuser is white.
But Maycomb is unfair in other ways too. With such a small town mentality, this is no place for an outsider, anyone a little different from the norm. A good example of this would be Boo Radley. The townsfolk have turned him into a scary boogie-man on the basis of gossip and hearsay. And yet he ultimately proves himself to be a kind and gentle man, a complete contrast to the terrifying urban legend spun by ignorant gossip-mongers.